As means for safely and economically transporting and storing a raw material gas such as natural gas or methane, a method using gas hydrate which is solid hydrate of the raw material gas has attracted attention recently. In general, the gas hydrate is generated under high pressure and low temperature (for example, 6.0 MPa and 4° C.). In an existing method for generating the gas hydrate, a raw material gas is supplied in the form of fine bubbles to raw material water, and the gas-liquid contact therebetween is carried out. In particular, a method is disclosed which uses a double-walled-pipe heat exchanger as a device for generating gas hydrate (for example, see Patent Document 1).
Here, FIG. 8 shows one example of a device 1X for generating gas hydrate (hereinafter, referred to as a generation device) in a device for producing gas hydrate. The generation device 1X includes a multi-pipe heat exchanger, and has therein a coolant circulation region 3, a branching chamber 20, multiple reaction pipe lines 2X, and a merge chamber 21. Moreover, the generation device 1X has a coolant flow inlet 22 and a coolant flow outlet 23.
Here, when raw material water w and raw material gas g are supplied to the generation device 1X, the raw material water w and so forth are distributed to the multiple reaction pipe lines 2X by the branching chamber 20. In this reaction pipe lines 2X, the raw material water w and the raw material gas g react with each other by gas-liquid contact to form a gas hydrate slurry h. In other words, the reaction pipe lines 2X are used as reactors. The flows of the gas hydrate slurry h generated in the reaction pipe lines 2X, respectively, merge with each other in the merge chamber 21, and the gas hydrate slurry h is discharged to the outside of the generation device 1X.
In parallel with the above, a coolant c is supplied from the coolant flow inlet 22 of the generation device 1X. This coolant c circulates in the coolant circulation region 3, cools the multiple reaction pipe lines 2X, and is discharged from the coolant flow outlet 23. In other words, the coolant c removes heat of formation of the gas hydrate.
The above-described generation device 1X has a structure in which the reaction pipe lines 2X through which the raw material water w and the raw material gas g flow are cooled, so that the heat of formation of the gas hydrate h is removed, and the generation rate of the gas hydrate h is improved. Note that when the temperature in the reaction pipe lines 2X is raised by the heat of formation of the gas hydrate h, the generation efficiency of the gas hydrate h decreases.
Next, FIG. 9 shows a generation device 1Y constituted of a double-walled-pipe (single pipe) heat exchanger. This generation device 1Y has a single reaction pipe line 2Y, a coolant circulation region 3, a coolant flow inlet 22, and a coolant flow outlet 23. The generation device 1Y also generates gas hydrate as in the case described above.
However, any of the gas hydrate generation methods has several problems. A first problem is that it is difficult to operate the device for producing gas hydrate continuously for a long term, because the gas hydrate is adhered to and grows on the inner walls of the reaction pipe lines to block the reaction pipe lines. Causes of the blocking are as follows: (a) the gas hydrate tends to accumulate at stagnation where the flow is weak; (b) the gas hydrate is generated on or adhered to cooling surfaces of the reaction pipe lines; and the like.
A second problem is that it is difficult to increase the production amount of the gas hydrate. In a device for producing gas hydrate with a large production scale of the gas hydrate, it is necessary to produce a large amount of the gas hydrate in a short term by increasing the generation efficiency of the gas hydrate. In order to improve the generation efficiency of the gas hydrate, it is important to improve the efficiency of the removal of the heat of formation. For this improvement, it is desirable to employ a countermeasure which enables prevention of decrease in the heat transfer coefficient of the heat exchanger for removing the reaction heat.
Moreover, factors which inhibit increase in the amount of the gas hydrate generated are as follows: (a) the gas hydrate generated at gas-liquid interfaces covers bubbles and acts as a diffusion resistance; (b) the bubbles of the gas merge with each other in the reaction pipe lines, so that the area of the gas-liquid surface is reduced; (c) a conventional micro bubble generation apparatus cannot generate micro bubbles, but generates bubbles with large diameters, when the amount of the raw material gas is large, and the gas-liquid ratio is high; and the like. In order to improve the generation efficiency of gas hydrate, it is desirable to employ a countermeasure which solves these problems, and which increases the contact interfaces of the bubbles.